Wireless phones have enabled mobile communications over wide geographic areas. A wireless phone is not constrained to any particular geographic location, but is only limited by the coverage areas supported by a compatible communication system. Many different systems have been established for wireless communications. Wireless phone systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), orthogonal frequency division multiplexing (OFDM), frequency modulation (FM), or some other modulation techniques. Some communication systems and some wireless phones are able to operate using one or more of the modulation techniques. Each of the different communication systems may operate in accordance with one or more distinct communication standards.
A wireless phone may be configured to operate in a Universal Mobile Telecommunications System (UMTS). The UMTS may be a third generation or higher wireless communication system such as a third generation CDMA wireless communication system or a Global System for Mobile communications (GSM) wireless communication system. There may be one or more specifications for a UMTS.
For example, a wireless communication device may be designed to support one or more GSM standards, such as the standards offered by the “3rd Generation Partnership Project (3GPP)” and embodied in the 3GPP specification where the term “3GPP specification” refers to all GSM specifications including those covering GSM Evolution (GSM EDGE), Global Packet Radio Service (GPRS), and GSM 3G, including implementations for a Universal Mobile Telecommunications System (UMTS). The 3GPP specification is provided in a set of documents provided by Technical Specification Groups (TSG) including specifications in series 01 through 13 for GSM only prior to Rel-4, series 21 through 35 for 3G/GSM R99 and later, and series 41 through 52 for GSM only for Rel-4 and later.
Nearly all of the wireless communication systems support some type of handoff or hand over. In a handoff, a mobile wireless terminal, also referred to as user equipment (UE), user terminal, mobile device, mobile unit, mobile terminal, wireless device, or wireless phone, senses that it is nearing the edge of a coverage area supported by a first transceiver. However, there may be a second transceiver that can provide a higher quality of service for communications with the user equipment. Thus, the communication system hands off the communication with the user equipment from the first transceiver to the second transceiver.
Hand off, or hand over as it is commonly referred, may be accomplished in a variety of ways. One method of handoff is termed a “hard handoff”. In a hard handoff, the communication link between the first transceiver and the user equipment is broken before the communication link is established between the second transceiver and the user equipment. Typically, the period in which the user equipment has no communication link to the system is short enough to be not noticeable by a user during a call.
A second method of handoff is termed a “soft handoff”. In a soft handoff, or soft hand over, the user equipment establishes communication with the second transceiver before the communication link to the first transceiver is broken. Thus, in a soft hand over the user equipment communicates simultaneously with two transceivers. The transceivers involved in the soft hand over transmit duplicate information to the user equipment. For example, during a voice call, the two transceivers transmit identical voice call information to the user equipment. The user equipment may be able to combine the signals from the two transceivers to create a single higher quality signal because identical information is being sent from two transceivers.
A third method of handoffs is termed “softer handoff”. Softer handoff, or softer hand over, refers to a soft hand over between multiple sectors of a single transceiver. In softer hand over, a single transceiver may provide a coverage area using multiple antennas. Each of the antennas may provide coverage for a sector within the aggregate coverage area supported by the single transceiver. A softer hand over essentially operates identically to a soft hand over, except that the hand over is between different sectors and not different transceivers.
User equipment typically continually searches for and synchronizes with signals from neighboring transceivers such that when a hand over is initiated, the user equipment is able to communicate with the new transceiver. Additionally, the user equipment may assist in the process of determining when a soft hand over should be initiated and the transceiver to which the communication link should be handed. Thus, the user equipment not only needs to continually search for and synchronize with neighboring transceivers, the user equipment also needs to continually evaluate the signals from neighboring transceivers in order to assist in determining whether a hand over should occur.
Thus, during a soft hand over and a softer hand over the user equipment simultaneously communicates with two different signal sources. Each of the signal sources transmits some information that may be identical. The user equipment may combine the two signals to create a higher quality combined signal. However, the two transmitted signals originate from two different sources. The two signal sources may be different antennas or different transceivers. The two signal sources may not be identically synchronized. That is, a first transceiver may transmit a signal to the user equipment slightly ahead of the time the second transceiver transmits the identical signal. Additionally, the propagation paths from the first transceiver to the user equipment are likely different from the propagation paths from the second transceiver to the user equipment. Thus, even if the signals from the first transceiver were synchronized with the signals from the second transceiver at the time of transmission, the signals may still arrive at the user equipment at different times due to path differences.
Thus, what is needed is a manner of determining the timing offset of the signals received from the second transceiver relative to the signals received from the first transceiver. In asynchronous wireless communication systems, such as systems covered under the UMTS Terrestrial Radio Access Network (UTRAN) or 3GPP specifications, the transmitted signals are configured as frames. Each of the frames has a corresponding frame number. It is desirable to unambiguously determine frame numbers of channels that are being set up for a soft hand over. It is also desirable to determine a time offset of the signals from sources that may be used in a soft hand over.